1. Field of the Invention
The invention relates to small joint orthopedic implants and, more particularly, to a technique for the manufacture of such implants that is exceedingly efficient and which eliminates the need for a hospital or surgeon to acquire and maintain an inventory of such implants.
2. Description of the Prior Art
Current methods of small joint orthopedic implant manufacture involve the fabrication of a universal set of sizes of implants, which are sold or consigned to hospitals along with accompanying instrumentation. Because the implants are manufactured to pre-determined specifications, the surgical procedure necessarily requires that the patient's bone be configured to fit a standard implant. This procedure has a number of drawbacks.
For example, in the case of radial head (elbow joint) surgery, “monoblock” radial head metallic implants have evolved. Due to their size and rigidity, these “off-the-shelf” implants often require excessive bone resection to allow their insertion and are technically demanding to use. The difficulties associated with monoblock implants have led to the development of modular implants in which differently sized heads are fitted to differently sized stems. The heads usually are provided in a limited number of radial sizes, e.g., small, medium, and large. Unfortunately, as the length of the head increases, typically so does the diameter, and it is unlikely that any combination of modular parts will perfectly fit a given patient. A skilled surgeon can adapt such off-the-shelf implants to fit the anatomy of an acute fracture, but it is not uncommon that even in experienced hands an implant does not achieve a desired level of stability. With all of the referenced implants, it usually takes a great deal of time and skill on the part of the surgeon to properly prepare the bone so that it will receive the implant properly in order to produce a strong, reliable result.
One form of radial head implant is known as the Swanson titanium radial head implant and is commercially available from Wright Medical Technology of Arlington, Tenn. In this implant, ten sizes of stems are provided (five “regular” width and five “narrow” width) and five head sizes are provided. The implants are accompanied by a sizing set made of a non-sterile plastics material. In order to install the implant, the surgeon enlarges the intramedullary canal of the radius using a curette, rasp, or drill. Implants from the sizing set are used to determine which head and stem should be implanted. Thereafter, a titanium stem and head corresponding to those of the sizing set are installed. After installation, the stem and head are fixed relative to each other, i.e., there is no rotation of the head relative to the stem.
There are a number of techniques whereby implants are sized to fit a particular patient, but such techniques are exceedingly difficult and expensive to accomplish. Generally, such techniques are suitable only for manufacturing implants for large, complexly shaped joints such as the hip or knee joints. For example, U.S. Pat. No. 4,704,686 to Aldinger discloses a method of producing an “individually adjusted” endoprosthesis pin, the method including processing data of the bone density of the patient's bone at a number of points. Such bone density data are obtained by X-rays, tomography or nuclear spin resonance. These imaging techniques are also used to obtain dimensional measurements of the subject bone or joint and to fabricate the implant.
U.S. Pat. No. 4,936,862 to Walker, et al. discloses a method for making joint prostheses that is semi-standardized and semi-customized. A mathematical model of average joints and bones is developed from a statistically significant sample of the population. Images of the patient in question at the operative site are taken by tomography or radiography. Based on the actual patient data, an appropriate standard model bone/joint is chosen from the population sample. This model is modified appropriately to more closely match the patient in question. A computer-generated shape is modeled by CAD and fabricated by CAM and/or CNC to produce a prosthesis that closely approximates the patient's bone. This method is essentially a compromise, providing a virtually customized bone implant, but which avoids the difficulties and cost of attempting to determine the shape of the patient's bone, both internally and externally, by CAT scan and duplicating the bone.
U.S. Application Publication 2003/0120276 to Tallarida et al. discloses a method for measuring and mapping the articular surface of a patient's bone or joint, and fabricating a prosthesis based on these data. A fixation screw is drilled into a bone surface to provide a reference axis. A static post is inserted into the reference axis as a measuring tool. The joint is articulated around this axis, and this motion is recorded. The motion is mapped by feeding it into parametric engineering design software, which defines a three-dimensional surface, that is, patient-specific measurements. An implant is fabricated to match the contours of this three-dimensional surface. Preparation tools also can be fabricated using the same dimensions obtained through the mapping procedure. Hence, implant geometry and preparation tool geometry can be mated and optimized allowing an implant of minimum thickness and a minimal removal of bone matter. Other methods of mapping the intended surgical site and/or articular surface are contemplated, including MRI or CT scanning.
U.S. Pat. No. 5,002,579 to Copf et al. discloses a joint prosthesis and method for making the prosthesis. A small number of X-ray images are taken of the subject bone or joint, and the major contours of the bone are determined and stored in a computer. A computer is loaded with a pre-existing database of three-dimensional geometric data of various thighbones. The computer finds the bone data in the database most similar to the subject bone data gleaned from the X-ray images. These data are transferred and the details of the subject bone are interpolated therefrom. A model then is made that corresponds to a best-fit prosthesis suited for the subject bone. A computer program also is used to allocate support posts of the prosthesis along trabecular spicules in order that lines of force transmission through the prosthesis correspond to those in nature.
U.S. Pat. No. 5,522,900 to Hollister and U.S. Pat. No. 5,549,690 to Hollister et al. collectively disclose a generalized prosthetic joint and a prosthetic thumb joint, respectively, and methods for their manufacture. The method of manufacture begins with modeling the prosthetic joint by determining the two non-perpendicular and non-intersecting axes about which the joint rotates. Rotation about these axes produces the outline of a torus. Portions of this torus are then chosen which form ideal load-bearing surfaces of the prosthetic joint. An aspect of the inventions is to not only mimic the kinematics of a joint, but also to mimic the natural bony structure of the joint.
As will be apparent from the foregoing, there remains a need for a technique to manufacture small joint implants that avoids the need for a surgeon or hospital to maintain a large inventory of standardized implants. There also is a need for a technique to manufacture small joint orthopedic implants without incurring the time and expense needed to manufacture large, complexly shaped implants. Moreover, any such technique would produce an implant having the capability for the head to rotate relative to the stem, and for axial adjustments of the head relative to the bone to be made easily during installation. Yet additionally, any such technique would permit the implant to be installed quickly and accurately, while avoiding imprecise, difficult, and time-consuming preparation time.